1. Field of the Invention
The present invention relates to a recordable optical disc on to which signals can be recorded by irradiating recording light from the transparent substrate side thereof, and in particular to an optical disc, on the surface of which is adhered a cover sheet, at the transparent substrate side thereof, i.e., a light irradiation or incident surface.
2. Description of Prior Art
Optical discs, such as a compact disc (i.e., CD) used as a medium for use in music and a CD-ROM used as a medium mainly for use in a computer and so on, both of which are available in the market, are media of read only (i.e., it is only possible to read out the signal or information from them). On the read only optical discs, ordinarily, signals are recorded by forming so-called pits of concave and/or convex shapes on a surface of the above transparent substrate, with the use of a pattern or mold which can be obtained through an electric casting process or press process, etc., when forming the transparent substrate from a polycarbonate or the like. With such an optical disc, however it is impossible for a user to record information by him/herself.
In contrast to this, from the end of 1980's to the beginning of 1990's, a so-called CD of the Write-once type (i.e., CD-R) was developed, which is recordable and compatible with the above CD. With this CD-R, ordinarily, it is shipped without recording the signals thereon, and the user can record arbitrary information on it at his/her desire only once. Thereby, it is possible for the user to obtain a copy or reproduce a CD or CD-ROM on which the arbitrary information is recorded, and which is compatible with such a CD or CD-ROM, at a relatively low cost.
Ordinarily, the CD-R is formed with a disc-like transparent substrate of a transparent synthetic resin, of such as polycarbonate or the like, on which is formed a recording layer of a coloring matter or pigment, such as a cyanine pigment. Further thereon, there is attached a metal reflection layer of such as an Au film, on which is further provided a protection layer of an ultraviolet curing resin. On the side surface of the above transparent substrate on which the pigment recording layer is formed, there is provided a tracking guide of a spiral shape, having a pitch of 1.6 .mu.m for example.
When recording the signal onto such the CD-R, a spot of recording light ray which is pulse-modulated is irradiated onto the pigment recording layer, while tracking the spot of the recording light ray onto the tracking guide, which is irradiated from an optical pickup through the transparent substrate onto the pigment recording layer, thereby writing the pulse signal as pits onto it. In this instance, the pigment material of the pigment recording layer being heated to melt and partially decompose corresponding to the pulse signal interacts with the resin material of the transparent substrate being heated and melted, mutually, thereby forming the pits on the transparent substrate. With this, desired information signal which can be read out in the same manner as in the CD and the CD-ROM can be written onto it.
With this CD-ROM, it is possible to obtain an optical disc, on which information completely compatible with the CD and the CD-ROM can be recorded with ease on a desk-top environment, without large-scaled manufacturing facilities nor complicated manufacturing processes which are used in the manufacturing processes of such as the CD and the CD-ROM for use of read only.
The transparent substrate of the optical disc is easily attracts dust on the light incident surface due to the static electricity generated thereon, since it is made of a synthetic resin, such as polycarbonate or the like, and is easily scratched when receiving friction with hard things or shocks on it. As mentioned previously, the dust and/or the scratches attached upon the light incident surface of the transparent substrate disturbs the incidence of the laser light, thereby greatly deteriorating the characteristics, in particular in the wiring characteristic thereof.
For solving the above-mentioned problem, in accordance with the conventional optical disc, a detachable cover sheet is attached upon the surface of the transparent substrate on which the recording light and the reproducing light are incident to protect the surface of the transparent surface. For example, with the disc on which a transparent cover sheet is attached, recording and the reproducing are possible by attaching the cover sheet upon the surface of the transparent substrate. While in a case where it is attached with a colored sheet, which has an effect of cutting off the light of the light sensitive wavelength region of the pigment recording layer or which is opaque or non-transparent, it has also an effect of protecting from deterioration the pigment recording layer due to the natural light, etc. Further, with the cover sheet containing an anti-static agent therein, it is possible to protect from adhesion of the duct due to the static electricity. In this manner, by attaching the cover sheet upon the surface of the transparent substrate of the recordable optical disc such as the CD-R or the like, there can be obtained various advantages.
By attaching the cover sheet upon the surface of the transparent substrate, however, there also can be caused disadvantages. A representative example of the disadvantages is a curving or warping of the optical disc. For instance, according to a so-called Orange Book which describes a standard for the CD-R, an allowable maximum angle of curving for the disc is determined to be equal or less than 0.6 deg. If the optical disc is curved by attaching the cover sheet on it, and if the curved angle exceeds that of the standard, it may cause recording errors and/or reproducing errors.